Long, narrow rivers of stars surrounding the Milky Way have been considered promising places to search for clues about mysterious dark matter.
But new research suggests that some of those clues may actually be produced by the gravity of our own galaxy.
The study, led by astronomers at the University of Washington, challenges the idea that unusual gaps, bends and clumps in these “stellar streams” necessarily point to hidden concentrations of dark matter.
The findings were published Aug. 27 in The Astrophysical Journal.
Dark matter is thought to account for most of the universe’s matter, yet scientists still do not know what it is. It does not produce or reflect light, making it invisible to telescopes. Its presence is instead detected through its gravitational effects on stars, galaxies and other visible objects.
Stellar streams have become one of the tools astronomers use to search for those effects.
These streams can form when groups of stars are captured by a galaxy’s gravity. As they orbit the galaxy, gravitational forces gradually stretch them into long, thin trails containing large numbers of stars.
Many of the stellar streams around the Milky Way are not perfectly smooth. Instead, astronomers see gaps, kinks, branches and other unusual structures. One popular explanation is that invisible clumps of dark matter, known as subhalos, pass near the streams and disturb the stars with their gravity.
The new study investigated whether the Milky Way itself could produce similar features without the help of dark matter subhalos.
Researchers created computer simulations of four galaxies similar in size to the Milky Way. They deliberately left out dark matter clumps and then placed around 15,000 simulated stellar streams inside the galaxies.
After allowing the virtual systems to evolve for five billion years, the researchers discovered something surprising: nearly every stream developed some kind of irregularity.
The reason was that galaxies are not perfectly smooth structures. Stars and other material are distributed unevenly, creating regions with slightly stronger or weaker gravitational forces. When stellar streams traveled through these areas, the changing gravitational pull could bend, stretch and disrupt them.
The simulations produced many of the same features astronomers observe in real stellar streams, including gaps, clumps, wiggles, branches and spurs. Some streams were even torn apart completely.
Streams passing closer to the center of their galaxies experienced the greatest disruption because they encountered denser and more complicated environments. Of the approximately 15,000 simulated streams, only 70 remained completely smooth after five billion years.
The findings do not mean that dark matter is absent or that stellar streams are useless for studying it. Instead, scientists may need to separate the effects caused by ordinary galactic gravity from those created by dark matter.
The researchers next plan to add dark matter subhalos to their simulations to determine whether they leave distinctive signatures that can be distinguished from the effects of the galaxy itself.
New observations from the Vera C. Rubin Observatory could provide an important test. Its powerful telescope is expected to discover many more stellar streams around the Milky Way, giving astronomers a much larger collection to study.
By combining those observations with increasingly realistic simulations, scientists hope to identify which stellar disturbances truly carry the fingerprints of dark matter.
Source: University of Washington.
